Multifunctional devices and digital cameras of recent years transmit large volumes of digital signals at a high speed in response to demands for a higher speed and a higher brilliance. To transmit the large volumes of data at a high speed, the number of transmission paths is increased or a transmission speed is increased. For the former case, the number of transmission paths that can be increased is limited in a downsized and high density electronic printed wiring board. Also, in the case of the transmission via a cable, an increase in the wiring number of cables directly causes a cost increase. Furthermore, a fluctuation in timings between signals is conspicuous by a skew along with an improvement in the transmission speed, and it is difficult to satisfy setup/hold. For that reason, a differential serial transmission system capable of transmitting the large volumes of data at a high speed with the fewer transmission paths is being widely used. In the differential serial transmission system, low speed parallel signals such as data, addresses, and control lines from a transmission side IC are serialized and output as differential signals to the transmission paths, and the serial signals as the transmitted differential signals are deserialized on a reception side IC to be converted into the parallel signals. It is noted that clock signals transmitted from the transmission side IC to the reception side IC may be of a clock embedded-type transmission system in which the clock signals are included in the above-described parallel signals to be serialized and transmitted or a transmission system or a clock independent-type transmission system in which only the clock signals are transmitted separately, apart from the above-described parallel signals.
On the other hand, in a case where the high speed signals are transmitted on a long lossy transmission path such as the cable, electromagnetic waves are emitted by harmonic components included in the differential signals while the cable functions as an antenna, which may affect operations of other devices. For that reason, electromagnetic interference (EMI) from the device is to be suppressed.
Also, the data signals are transmitted by way of rectangular waves in the serial transmission. FIG. 8A conceptually illustrates a spectrum of a normal mode component of the transmitted signal. As may be understood from FIG. 8A, the spectrum of the normal mode component is represented by a sinc function. Here, in the serial transmission at a high speed, a bare minimum spectrum of the normal mode for data reproduction on the reception side IC is a first envelope part in FIG. 8A.
On the other hand, FIG. 8B conceptually illustrates a spectrum of a common mode component of the transmitted signal. The common mode component is a component that causes a problem as radiation noise, which is generated from unbalance of the differential signals. In FIG. 8B, when a frequency is at 500 MHz, 1 GHz, and 1.5 GHz, very large signals of the common mode component are generated. These frequencies at 500 MHz, 1 GHz, and 1.5 GHz are frequencies where the signal of the normal mode component becomes zero in FIG. 8A. That is, at the frequency where the spectrum of the normal mode signal illustrated in FIG. 8A disappears, the signal of the common mode component with a large amplitude is generated.
It is described in PTL 1 that a band limiting on the data spectrum is carried out by using a low-pass filter (LPF) so that an intersymbol interference does not occur because a frequency band provided to the transmission path used for the serial transmission is not regarded as indefinite.
FIG. 8C illustrates a simplified configuration of the LPF described in PTL 1. In FIG. 8C, one of the differential serial signal transmission paths is structured by an inductor 51 and a pair of pi-type filters composed of two capacitors 53 and 55. In addition, the other differential serial transmission path is structured by an inductor 52 and a pair of pi-type filters composed of two capacitors 54 and 56. This LPF is inserted between a differential signal transmission unit and a differential signal reception unit, and a cutoff frequency is set as a maximum frequency of the first envelope, so that a frequency component higher than the first envelope is removed. According to this, a high-pass unused spectrum is removed with respect to the normal mode component, and a high-pass unused spectrum is removed at the same characteristics also with respect to the common mode component.
However, in the above-described related art LPF, the common mode component is also suppressed at a performance equivalent to the suppression of the normal mode component in the unused high frequency band, but the common mode component that becomes the cause of the radiation noise also exists in a frequency band lower than or equal to the cutoff frequency of the LPF. To elaborate, in the related art LPF, since the suppression with respect to the common mode component in the frequency band of the first envelope is insufficient, a further improvement in the LPF is sought after.